1. Technical Field
The present invention relates to a numerical control apparatus that can control a machine tool. More specifically, the present invention relates to a numerical control apparatus that has a vibration suppression function capable of suppressing chattering vibrations that may occur when a tool or a workpiece rotates about a rotational center thereof to process the workpiece.
2. Related Art
For example, a widely known conventional machine tool includes a rotary shaft that can support a tool and move the tool relative to a fixed workpiece to perform cutting along a circumference surface of the workpiece. In the cutting process of the machine tool, if the depth of cut or the protrusion length of the tool is excessively large, so-called “chattering vibrations” occur during the course of processing and deteriorate the finishing accuracy of a surface to be processed.
In this case, a method capable of suppressing the “chattering vibrations” is conventionally known. The conventional method includes obtaining a natural frequency of a system (e.g., a tool or a workpiece) in which the “chattering vibrations” may occur or a chattering frequency arising during the course of processing. The method further includes multiplying the obtained frequency (i.e., the natural frequency or the chattering frequency) by 60 and dividing the calculated value by the number of tool flutes and a predetermined integer to determine a target rotational speed to be used in the processing. There is a conventional vibration suppression apparatus capable of obtaining an optimum rotational speed at which the “chattering vibrations” can be effectively suppressed.
For example, as discussed in JP 2009-101495 A, a machine tool has a rotary shaft that causes a tool or a workpiece to rotate about a rotational center thereof and the machine tool is equipped with a vibration suppression apparatus capable of suppressing chattering vibrations that may occur when the rotary shaft is rotating. The vibration suppression apparatus includes a detection unit that detects a time domain vibration of the rotary shaft while the rotary shaft is rotating, a first calculation unit that calculates a chatter frequency and a frequency domain vibration of the chatter frequency based on the time domain vibration detected by the detection unit, and a storage unit that stores machining information including the frequency domain vibration, the chatter frequency, and a rotary shaft rotational speed.
If the frequency domain vibration calculated by the first calculation unit exceeds a predetermined threshold, a second calculation unit acquires new machining information including a frequency domain vibration, a chatter frequency, and a rotary shaft rotational speed at this moment, and stores the acquired new machining information in the storage unit. The second calculation unit calculates an optimum rotational speed of the rotary shaft that can suppress chattering vibrations, by reference to the new machining information and the past machining information stored in the storage unit. The vibration suppression apparatus further includes a rotational speed control unit that causes the rotary shaft to rotate about a rotational center thereof at the optimum rotational speed calculated by the second calculation unit.
Further, in the conventional vibration suppression apparatus, the optimum rotational speed calculated in this manner is recorded in an optimum rotational speed recording unit together with a tool number of the tool supported on the rotary shaft when the optimum rotational speed is calculated and a command rotational speed of the rotary shaft included in a machining program.
Thus, when the processing is performed again by means of a tool having the same tool number and at a command rotational speed identical to that in the optimum rotational speed calculation, it is feasible to perform the processing at the previously calculated optimum rotational speed (a substitute for the command rotational speed) recorded together with the command rotational speed. However, even if the tool having the same tool number and the recorded optimum rotational speed are used, the “chattering vibrations” may fail to be suppressed sufficiently, because the cutting resistance acting on the tool varies due to, for example, abrasion of the tool.
In general, only one optimum rotational speed exists within a corresponding rotational speed range. Therefore, in a case where the processing is performed by means of the same tool at a plurality of command rotational speeds, if all of the plurality of command rotational speeds fall within only one type of rotational speed range, only one optimum rotational speed exists.
On the other hand, if the plurality of command rotational speeds fall within a plurality of types of rotational speed ranges, a respective optimum rotational speed exists within each of the plurality of types of rotational speed ranges. More specifically, in each tool, a plurality of types of optimum rotational speeds are recorded for a plurality of command rotational speeds. The number of the types of the optimum rotational speeds is different from the number of types of the command rotational speeds.
A conventional method for substituting a command rotational speed designated in the machining program by an optimum rotational speed, to perform processing at the optimum rotational speed recorded in the above-described manner, is described in detail below.
FIG. 1 is a block diagram illustrating an example of a numerical control apparatus that includes a conventional vibration suppression function. In FIG. 1, a program analyzing unit 19 extracts a command rotational speed CMD-S from a machining program 18 to cause a rotary shaft 10 holding a tool 11 to rotate about a rotational center thereof. The program analyzing unit 19 supplies the extracted command rotational speed CMD-S to a command rotational speed substitutability determination unit 17. Further, the command rotational speed substitutability determination unit 17 receives an optimum rotational speed LOG-S from an optimum rotational speed recording unit 16.
The command rotational speed substitutability determination unit 17 determines whether the command rotational speed CMD-S can be substituted by the optimum rotational speed LOG-S. If it is determined that the command rotational speed CMD-S can be substituted by the optimum rotational speed LOG-S, the command rotational speed substitutability determination unit 17 supplies the optimum rotational speed LOG-S, as a command rotational speed CMD-S′, to a rotational speed control unit 15.
Next, a method for determining whether the command rotational speed can be substituted by the optimum rotational speed is described below. FIG. 3 is a flowchart illustrating an operation that can be performed by the conventional command rotational speed substitutability determination unit 17.
In FIG. 3, in step S1, the command rotational speed substitutability determination unit 17 sets an initial value “0” for the command rotational speed CMD-S′ to be output to the rotational speed control unit 15. Next, in step S3, the command rotational speed substitutability determination unit 17 compares the command rotational speed CMD-S extracted by the program analyzing unit 19 with a command rotational speed (hereinafter, referred to as “corresponding rotational speed”) associated with the optimum rotational speed LOG-S recorded in the optimum rotational speed recording unit 16.
In the present example, the corresponding rotational speed is a command rotational speed at timing when the optimum rotational speed LOG-S is calculated. If, as a result of the comparison, it is determined that the command rotational speed CMD-S coincides with the corresponding rotational speed, then in step S4, the command rotational speed substitutability determination unit 17 sets the recorded optimum rotational speed LOG-S (i.e., the optimum rotational speed that corresponds to the corresponding rotational speed) as the command rotational speed CMD-S′.
In the present example, the optimum rotational speed recording unit 16 stores a plurality of optimum rotational speeds. Therefore, in step S2, the command rotational speed substitutability determination unit 17 confirms whether the processing of step S3 and step S4 has been completed for each optimum rotational speed LOG-S recorded in the optimum rotational speed recording unit 16.
In step S5, the command rotational speed substitutability determination unit 17 confirms whether the command rotational speed CMD-S′ to be output to the rotational speed control unit 15 is the initial value “0” at timing when the confirmation processing has been completed for all of the optimum rotational speeds LOG-S recorded in the optimum rotational speed recording unit 16.
If it is determined that the command rotational speed CMD-S′ is equal to the initial value “0”, then in step S6, the command rotational speed substitutability determination unit 17 sets the command rotational speed CMD-S designated in the program as the command rotational speed CMD-S′ to be output to the rotational speed control unit 15. Then, in step S7, the command rotational speed substitutability determination unit 17 determines that the command rotational speed is not substitutable.
If in step S5 it is determined that the command rotational speed CMD-S′ to be output to the rotational speed control unit 15 is not the initial value “0”, then in step S8, the command rotational speed substitutability determination unit 17 determines that the command rotational speed is substitutable, because the recorded optimum rotational speed LOG-S is set as the command rotational speed CMD-S′ to be output to the rotational speed control unit 15.
According to the above-described conventional command rotational speed substitutability determination, although only one optimum rotational speed is present within a rotational speed range, the command rotational speed is substituted by an optimum rotational speed corresponding to the corresponding rotational speed only when a command rotational speed of the machining program coincides with the corresponding rotational speed recorded in the optimum rotational speed recording unit.
In other words, if the command rotational speed designated in the machining program is slightly different from the recorded corresponding rotational speed, the numerical control apparatus does not perform substitution processing. As a result, the recorded optimum rotational speed cannot be used. Therefore, even in a case where a plurality of corresponding rotational speeds are stored in the optimum rotational speed recording unit, the recorded optimum rotational speeds cannot be used for various command rotational speeds (e.g., command rotational speeds having not yet been used). Then, if the machining program designates a command rotational speed that is different from the previously used value, the numerical control apparatus cannot substitute an optimum rotational speed for the command rotational speed even when the same tool is used to perform processing. As a result, chattering vibrations may occur. In this case, the finishing accuracy of a surface to be processed deteriorates significantly.
Hence, the present invention intends to provide a numerical control apparatus that can suppress chattering vibrations effectively.